Shaft couplings are often used to connect opposed, rotatable shafts in end-to-end relationship. The torque developed by the driving shaft is transmitted to the driven shaft through the coupling, as, for example, from a motor to a fan blower unit. This allows synchronous power transmission without the need for an additional motor, greatly lessens the cost of operation, and simplifies the motor control system. The coupled shafts must be precisely aligned in both the vertical and horizontal planes to effectively transmit the developed torque, to minimize wear on the couplings, seals, shafts, and bearings, and to minimize energy consumption, which increases greatly where misalignment is present and as it becomes progressively worse. Precise shaft alignment is difficult to obtain and even more difficult to maintain under operating conditions; however, for optimum performance, the center lines of the shafts should be both concentric and coinciding when the equipment reaches equilibrium operating condition.
Misalignment of the shafts can be angular where the projected center lines of the shafts intersect at an angle, or it can be parallel offset where the projected center lines of the shafts are parallel but are not concentric. These conditions may also exist simultaneously, and the shafts may exhibit both parallel and angular misalignment. Factors which may cause misalignment include uneven settling of the foundation beneath the components, expansion of the shafts or other affected elements due to the heat of operation, shaft deflection, wear on the coupling or in the bearings which affects clearance, and dimensional changes in the structural and rotating elements. Slight and unavoidable misalignments can be compensated for by the use of flexible couplings; however, these are not designed as substitutes for proper shaft alignment procedures. Operation of the machinery while misaligned causes early wear and, ultimately, failure of the flexible couplings also, especially in the flexible element itself.
Shaft alignment procedures are widely known and practiced in industry, and several methods are currently in use. Examples include the reverse indicator method, where measurement readings are made from one shaft to the other and then back again, generally considered to be the most accurate method; the rim and face method, in which the coupling is disassembled, and two dial indicators mounted on a beam are used for measurements against the rim and face of the exposed hubs; and even a trial and error procedure, using a straight-edge and calipers, in which readings are taken as in the rim and face method without disassembling the coupling, and the required calculations and graphical representations are foregone in favor of slight trial movements of the machinery. One of the shafts to be aligned is considered to be stationary and the other shafts are aligned to the stationary unit. Most current methods involve mounting a clamp on the fixed shaft, using upper and lower clamp halves connected by chains or long bolts which approximate the diameter of the shaft. Extending outwardly from the clamp is a rod, which connects to a second rod that extends horizontally across the coupling. A third rod is then extended from the second rod to the shaft to be aligned, and measurement indicators reflect the degree of misalignment in the horizontal and vertical planes. Readings are taken from the indicators, normally at ninety degree intervals, and the results are either plotted graphically or fed into a specially programmed computer. Either of these methods can be used to calculate the amount which the machine or component must be moved to align the shaft with the fixed reference shaft.
In some cases, clamps or similar members with measurement devices are secured to the coupling hubs, rather than to the shafts. The hubs mounted to the ends of the shafts are designed in many different shapes and sizes, and while some features are common to such hubs, in general, each manufacturer supplies a hub with a different external configuration. This raises some problems in attempting to mount an alignment device to the coupling hubs without modifying and/or standardizing certain features of the hubs to accept the alignment system. Even where possible, the cost of changing or modifying the hubs may be prohibitive, and down-time of the affected machinery substantial. Shaft-mounted systems may also suffer certain disadvantages. If there is a limited amount of available space on the shaft to mount the clamps, the coupling must be disassembled and removed before alignment can proceed. Replacing the coupling may then skew the alignment. Where large shafts are to be aligned, additional elements may be necessary, such as larger clamps and longer chains or bolts, requiring the stocking or availability of a plurality of different sized clamp members. Since the measurement apparatus must extend outwardly from the reference shaft to a point outside the coupling radius, and from there horizontally across the coupling to the second shaft, indicator sag effects, from the weight of the indicator on the extended rod, are normally a problem. While compensation factors for indicator sag effects have been developed, they are only an estimation and, therefore, introduce further uncertainty into the alignment equation. The cumulative effect of these disadvantages normally requires that several movements of the component to be aligned be undertaken before an acceptable shaft alignment is reached.